Remove the dead snails and do a water change. Check your water parameters.

What are the water parameters?
What test kit did you use?
Temperature?
Filtration?
Have you observed any unusual signs in your snails' behavior?
How long had you kept them?
Where did you buy them?
Did the dead snails smell badly when you found them?

Dang.....with any death make a 50% water change....have you added anything to the tank like algicide or any copper products.....or anything at all different...also, what about your source water....what is the source.....tap, well, bottled...etc...

Are they new snails? What size is the tank and what is the stocking? Did you acclimate them properly? Feed them enough?

With any livestock, sometimes the quality is just bad. If you didn't add anything and none of the other occupants are sick or acting strangely then a big water change and some close observation over the next few days is all you can do.

I lost 2 snails within a few weeks. I had purchased 5 from a private breeder and they all did well for a few months...then slowly I noticed that a few were growing nicely and the others were not. They died within a few weeks of eachother.

Just for reference, heavy metals tend to be more toxic at soft acidic conditions. I'm interested to know the pH and hardness levels as well. This is very important with snails. Snails suffer shell pitting in conditions not in their favor.

Do you guys think that is what it was? I changed the water. Tonight I am redoing my whole tank, so it would be a full 100% change, I am going to keep the bio-filter though, in some old tank water, so I don't have to restart the cycle.